react-native-webview : For iOS text too small

Additionally to the <meta> you can add <style> tag into the <head> section to scale the font:

<style>
    body { font-size: 120%; word-wrap: break-word; overflow-wrap: break-word; }
</style>

Using the viewport meta tag to control layout on mobile browsers

<meta name="viewport" content="width=device-width, initial-scale=1">

<WebView
      originWhitelist={['*']}
      source={{ html: '<html><head><meta name="viewport" content="width=device-width, initial-scale=1.0"></head><body><p>This is a static HTML source!</p></body></html>' }}
/>

link :https://github.com/react-native-community/react-native-webview/issues/386